The present invention relates to measuring the surface profile of a feature, and in particular to measuring the surface profile using a differential interferometer.
Differential interferometers are known in the art. For general information relating to differential interferometers, the reader is directed to xe2x80x9cLASSIxe2x80x94a scanning differential ac interferometer for surface profile and roughness measurementxe2x80x9d by G. Makosch, SPIE Vol. 1009, Surface Measurement and Characterization (1988), pp. 244-253, which is incorporated herein by reference. In a conventional differential interferometer, a beam of light, such as a laser, is decomposed into two orthogonally polarized beams, a reference beam and a measurement beam. The optical phase difference between the two beams is varied using a voltage-controlled modulator. Using beam splitting optics, e.g., consisting of a Wollaston-prism combined with a microscope, the two beams are focused onto and reflected back from a sample surface. On reflection, the two beams are recombined, e.g., using a Wollaston-prism, and partially reflected by a beam splitter to a photon detector. The detected total intensity, I, in a differential interferometer is given by the following equation:
I=I1+I2+2{square root over (I1I2)} cos(xcfx86xe2x88x92xcfx86M)xe2x80x83xe2x80x83eq. 1
where I1 and I2 are the respective intensities of the two beams, xcfx86 is the phase difference between the two beams due to the reflections on the sample and the optical path difference of the two beam in the optical system, and xcfx86M is the induced phase shift between the two beams introduced by the modulator.
Conventionally, a differential interferometer is used to determine the step height between two regions on a sample or the surface profile of the sample using:                     ϕ        =                              ϕ            2                    +                                    4              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              h                        λ                    -                      ϕ            1                                              eq        .                  xe2x80x83                ⁢        2            
where xcfx861 and xcfx862 are the phase shift of the reference beam and the measurement beam due to reflection, respectively, h is the step height between the two regions being illuminated, and xcex is the wavelength of light. The phase xcfx86 is measured by the differential interferometer. Conventionally, the sample is optically characterized prior to the differential interferometer to determine the reference beam phase xcfx861 based on the composition and thickness of the film stack and optical constants (n and k values), e.g., using a spectral reflectometer or spectroscopic ellipsometer, as described in U.S. Pat. No. 5,045,704, which is incorporated herein by reference. The optical characterization is done in a uniform region that serves as the reference region. In the beginning of the differential interferometer scan, both the reference and measurement beams sample the reference region. The measurement beam is then scanned across the measurement region. The reference beam phase xcfx861 is assumed to be constant during the scan. Thus, a profile of measurement phase xcfx862 versus step height h is produced. Because the measurement beam originated the scan from the reference region, the step height h difference between the reference region and the measurement region can be determined.
Unfortunately, the reference phase xcfx861 may not be a constant during the differential interferometer scan. For example, the reference beam may pass over a feature, e.g., a cavity or an object above the sample surface, during the scan, which will change the reference beam phase xcfx861. The region over which the reference beam passes, may also vary in thickness, which will again change the reference beam phase xcfx861. Consequently, the assumption that reference beam phase xcfx861 is uniform may be incorrect and may result in errors in step height measurements. Moreover, multiple solutions may be possible for equation 2, giving an ambiguous step height h. In addition, when the measurement beam scans over a composite layer, e.g., having two or more materials, such as copper lines embedded in a silicon dioxide layer, the step height h result will depend on the area fraction of the materials as well as any height differences between the two materials. If the area fraction or the height differences are not known, the resulting step height h or profile result will be unreliable.
It is therefore desirable to derive more information from the reference beam and measurement beam in a differential interferometer to achieve a more accurate step height measurement.
To accomplish the above and other objects, the present invention overcomes the difficulties of prior art approaches by using the reflectance from the reference and measurement beams in a differential interferometer measurement. The present method is particularly applicable to the samples where the reference and/or measurement regions are not opaque.
A differential interferometer is used to measure the step height between a reference region and at least one point in a measurement region using the phase difference as well as the measured reflectance from at least the point in the measurement region. The measured reflectance can be derived from the information provided by the differential interferometer. The measured reflectance from the reference region can also be used to provide the step height, where, e.g., the reference region has a changing thickness. Where the measurement region includes a composite material, e.g., copper and silicon dioxide, the step height between the reference region and the measurement region may be determined by including the area fraction and/or the height difference of the materials in the composite material in the final determination of the step height.
Thus, in accordance with the present invention, the step height between at least one point in a reference region and at least one point in a measurement region on a sample is measured using a differential interferometer. The phase shift at the reference region is determined, e.g., using a spectroscopic ellipsometer, reflectometer or a library. A differenfial measurement between the reference region and the measurement region is made to determine the measured relative phase shift between the at least one point in the measurement region and the at least one point in the reference region. The phase shift at the measurement region is calculated as a function of the thickness of the material in the measurement region. The measured reflectance from the measurement region is determined. The measured reflectance from the measurement region may be determined using the differential interferometer measurement, along with the known reflectance from the reference region and a differential interferometer measurement taken at the reference region, i.e., two points in the reference region. In addition, the reflectance from the measurement region is calculated as a function of thickness using known values of n and k. The step height between the reference region and the measurement region can then be determined using the phase shift of the reference region, the calculated phase shift from the measurement region, the measured relative phase shift, the measured reflectance from the measurement region and the calculated reflectance from the measurement region. For example, curve fitting can be used to determine the step height between the at least one point in the measurement region and the at least one point in the reference region.
Where the thickness of the material in the reference region changes, the phase shift of the reference region is calculated as a function of thickness, as opposed to being directly measured. The measured reflectance from the reference region is also determined, in a manner similar to the determination of the reflectance from the measurement region. In addition, the reflectance from the measurement region is also calculated as a function of thickness using known values of n and k. The step height is then determined also using the calculated phase shift from the reference region, as well as the measured and calculated reflectance from the reference region.
Where the material in the measurement region includes two different materials having an area fraction, the reflectance and phase from the measurement region is calculated as a function of thickness and area fraction. The calculated reflectance and phase from the measurement region is then used to determine the step height.
Where the material in the measurement region includes two different materials having a height difference, the reflectance and phase from the measurement region is calculated as a function of thickness and height difference. The calculated reflectance and phase from the measurement region is then used to determine the step height.